I'm mainly left-handed. Advantages:
* On conventional videogame controllers, the important part of the pad it the analogue. Even when there's two, the analogue stick on the left is generally more important. Left-handers have the advantage of using their main hand for the accuracy demanding left thumb-stick.
* When walking in groups of people, holding hands with girlfriends/boyfriends (walking around), etc:
Left handed people prefer to stand on the left, as their mind subconciously, and automatically tells you that it's safer to stand that side as you have your left-hand free to protect yourself. When in a group of people (who'll generally be right-handers), they'll automatically go stand on the right, leaving your subconciously favourite position open for you.
* Left generally seen as being the natural start. "From left to right" is more used than "From right to left" - most cultures write from left to right. Thus, left-handed is theorically more natural, in a sense.
* Individuality. Even people who lack egotism like their individuality. If being in a minority group doesn't cause any negative effects, then it is good for non-hardcore-mainstreamers.
Being left-handed does have some negatives, but I think that left-handers, overall, get the best deal.